In recent years, downsizing of engines equipped with a turbocharger has become common in the field of automobile engines, as a technique to reduce fuel consumption. Furthermore, in recent years, evaluation of the fuel economy performance of an automobile includes more test results obtained during extremely low-speed operation and acceleration of the engine, and thus there is an increasing need toward turbochargers that can exert a high turbine efficiency while the engine operates at an extremely low-speed. Thus, variable geometry turbochargers are becoming increasingly popular, which include a variable nozzle mechanism for controlling a flow direction of a fluid that flows toward a turbine impeller (turbine wheel). In this context, a turbine impeller are required to exert a high turbine efficiency in a wide operational range from extremely low-speed engine operation to high-speed engine operation, and to have a high transient responsiveness.
Generally, to improve the performance of a turbine impeller during low-speed operation of an engine, it is necessary to set the blade height and the throat area of the turbine impeller suitably for operation at a low flow rate. On the other hand, however, this leads to a decrease in the turbine efficiency during operation at a high flow rate.